1. Field of the Invention
The present invention relates to a light emitting element that comprises an anode, a cathode, and a layer containing an organic compound (hereinafter, electroluminescent layer) that generates light by applying electric field through the electrodes; and a light emitting device that comprises the light emitting element. Specifically, the present invention relates to a light emitting element that exhibits white emission and a full color light emitting device comprising the light emitting element.
2. Related Art
A light emitting element emits light when electric field is applied thereto. The emission mechanism is a carrier injection type. That is, by applying voltage through a pair of electrodes that interposes an electroluminescent layer therebetween, electrons injected from a cathode and holes injected from an anode are recombined within the electroluminescent layer to form molecules in excited states (hereinafter, excited molecule), and the excited molecules return to the ground state while radiating energy to emit photon.
There are two excited states possible from organic compounds, the singlet state and the triplet states. Light emission from the singlet state is referred to as fluorescence and the same from the triplet state is referred to as phosphorescence.
In such light emitting element, an electroluminescent layer is generally formed to have a thickness of below 1 μm. Further, since a light emitting element is a self-luminous element in which an electroluminescent layer emits photon, a back light used for the conventional liquid crystal display device is unnecessary. Therefore, a light emitting element has a great advantage of being manufactured to have a ultra thin film thickness and light weight.
In the case of an electroluminescent film with a thickness of approximately 100 nm, the time between the injection of carriers and their recombination is about several ten nanoseconds considering the carrier mobility. Hence, the time required for the process of injecting carriers and emitting light of the electroluminescent layer is on the order of microsecond. Thus, an extremely high response speed is one of the advantages thereof.
Further, since a light emitting element is carrier injection type, it can be driven by a direct current voltage, thereby noise is hardly generated. With respect to a drive voltage, an electroluminescent layer is formed into a uniform ultra thin film having a thickness of approximately 100 nm, and a material for an electrode is selected to reduce a carrier injection barrier. Further, a hetero structure (two-layers structure) is introduced. Accordingly, a sufficient luminance of 100 cd/m2 can be obtained at an applied voltage of 5.5V (reference 1: C. W. Tang and S. A. VanSlyke, Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
A light emitting element has been attracted attention as a next generation's device for a flat panel display in terms of the thin thickness and light weight, the high response speed, the direct low voltage operation, or the like. In addition, a light emitting element can be used effectively as the device for the display screen of a portable electric appliance in terms of the self luminous type, the wide viewing angle, and the high level of visibility.
Wide variations of emission color is also one of the advantages of a light emitting element. Richness of color is resulted from the multiplicity of an organic compound itself. That is, an organic compound is flexible enough to be developed to various materials by designing molecules (such as introducing substituent). Accordingly, a light emitting element is rich in color.
From these viewpoints, it would not be an overstatement to say that the biggest application areas of a light emitting element is a full color flat panel display device. Various means for full colorization have been developed in view of characteristics of a light emitting element. At present, there are three primary methods of forming the structure of a full color light emitting device by using a light emitting element.
First, the method that light emitting elements having three primary colors, that is, red (R), green (G), and blue (B) are patterned, respectively, by shadow mask technique to serve them as pixels (hereinafter, RGB method). Second, a blue light emitting element is used as a light emission source, and the blue emission is converted into green or red by color changing material (CCM) made from phosphorescent material to obtain three primary colors (hereinafter, CCM method). Third, a white light emitting element is used as a light emission source, and a color filter (CF) used for a liquid crystal display device or the like is provided to obtain three primary colors (hereinafter, CF method).
Of these methods, the CCM method and the CF method do not require such elaborate patterning required in the RGB method since a light emitting element used in the CCM method and the CF method exhibits single color such as white (CCM method) or blue (CF method). The CCM materials or color filter can be made by the conventional photolithography technique without complicated processes. Further, in addition to these advantages with respect to processes, the change in luminance with time of each color is uniform since only one kind of device is used.
However, in case of adopting the CCM method, there has been a problem in red color since color conversion efficiency of from blue to red is poor in principle. In addition, there has been a problem that the contrast becomes deteriorated since a color conversion material itself is fluorescent so that light is generated in pixels due to outside light such as sunlight. CF method has no such problems since a color filter is used as well as the conventional liquid display device.
Accordingly, although the CF method has comparative few disadvantages, the CF method has a problem that a high efficient white light emitting element is indispensable to the CF method since a great deal of light is absorbed into a color filter. A mainstream white light emitting element is the device that combines complementary colors (such as blue and yellow) (hereinafter, two wavelengths white light emitting device) instead of white color having the peak intensity in each wavelength of R, G, and B (reference 2: Kido et al., “46th Applied Physics Relation Union Lecture Meeting” p1282, 28a-ZD-25 (1999)).
However, considering a light emitting device combined with a color filter, a white light emitting element having an emission spectrum with the peak intensity in each wavelength of R, G, and B (hereinafter, three wavelengths white light emitting device) is desirable instead of the two wavelengths white light emitting device, which was reported in the reference 2.
Such three wavelengths white light emitting device has been disclosed in some references (reference 3: J. Kido at al., Science, vol. 267, 1332-1334 (1995)). However, such three wavelengths white light emitting device is inferior to the two wavelengths white light emitting device in terms of luminous efficiency, consequently, significant improvement is required. In addition, it is difficult for the three wavelengths white light emitting device to obtain stable white emission due to change in color with time or change in spectrum depending on the current density.